In the handling of such thick viscous stored materials heating is required to give effective product flow with minimal stickage. Of particular interest are the hard oxidized grades of bitumen which have to be handled at temperatures in excess of 200.degree. C. to enable effective product pumping and are transported in road or rail delivery tankers. After deliveries, particularly over long distances, a tare buildup of the order of 2 to 21/2 tons in 18 ton capacity delivery tankers can occur. The penalties associated with tare buildup are: reduction in carrying capacity of tankers; tanker sequencing and product contamination problems; fuel wastage in carrying unwanted extra weight. The removal of tare buildup by physically chipping out of the hardened product is a time consuming job resulting in the vehicle being out of usage for several days.
Also of interest are materials such as lube oil additives and base oils. It is usual to heat these products which are stored in drums to about 40.degree. C. before handling to give increased flow but exceptional care must be taken to ensure the material temperature does not exceed 80.degree. C. otherwise product degradation occurs.
Traditional heating methods (for example electric ovens or steam heating) are slow and not very efficient. Current industrial and domestic microwave heating techniques rely upon placing the product to be heated in ovens or cavities specifically designed to receive microwave energy and provide uniform heating. This is not practicable for the present application wherein a successful solution to the tare buildup problem would result in an unmodified bitumen tanker driving into a microwave bay and having a microwave waveguide connection made to the tanker in order to allow microwave energy to heat the bitumen tare. For the lube oil additives and base oils stored in drums the microwave energy would be directed into the drum via a waveguide connection, screwed directly into the filling bunghole. The immediate problems in both these applications are ensuring the heat distribution is uniform.
If a continuous electromagnetic wave at microwave frequencies is injected into a tank or drum a standing wave interference field pattern will be set up which will not vary with time. The pattern consists of a series of points of minimum intensity called nodes and maximum intensity called antinodes. The precise configuration of the standing wave or mode pattern is dependent upon the frequency of the microwave energy used to excite the cavity and upon the dimensions of the cavity itself. Since the degree of microwave heating is proportional to the applied field strength such a pattern gives rise to hot spots at the antinodes and little or no heating at the nodes.
In an effort to alleviate the problem of non-uniform energy distribution, a great many approaches have been tried. Two common techniques currently applied to avoid non-uniform heating are mode stirring or product movement. The former relies on the insertion of a large mechanically rotating fan to mix the waves as they enter the vessel thereby continually changing the standing wave pattern with time. It is not feasible to fit such a mode stirrer in a road or rail tanker or drum because of physical size limitations in inserting such a fan within the tanker or drum. The second technique accepts a fixed standing wave pattern and moves the product through it. Again this is not feasible for high viscous materials in road or rail tankers or drums.
U.S. Pat. No. 4,324,968 describes a different technique for promoting time averaged uniformity of microwave energy distribution. In this patent, a coupling aperture such as an X slot is used for radiating circularly polarized microwave energy from a feedwave guide into the adjacent heating cavity, where the slot is capable of being moved selectively with respect to the waveguide centerline. This movement of the X slot aperture causes a continual change in the sectional shape of the resulting field, from circular to elliptical with the degree and orientation of the ellipse depending upon the direction and degree of movement of the coupling aperture with respect to the waveguide centerline. Rather than physically moving the aperture, a device is provided for varying the electrical position of the coupling aperture with respect to the centerline of the waveguide as a function of time. In contrast, the present invention provides a mechanical means to continually vary linearly polarized microwave energy from a waveguide into an adjacent heating cavity. This continually changing microwave beam sweeps the interior of the heating cavity and thereby improves the time averaged energy distribution within the cavity. As a result of the continually changing feedwave, the standing waves in any one direction of the cavity dimensions are minimized.
It is therefore an object of the invention to provide a fast and efficient method and apparatus for uniform microwave bulk heating of high viscous materials in cavities such as road or rail tankers or drums which are not specifically designed for microwave heating.